Volume 1

Preamble • What Is Acidic Reacts with What Is Basic and What is Basic Reacts with What is Acidic • Reactions That Go to Completion • Exercises
1.15.1. Preamble. In the next sections of this Volume, you will be learning about quite a number of reactions involving various acids, bases, and salts. You may find the amount of information presented in these sections overwhelming and the information itself somewhat redundant and repetitive. While I strongly believe in the saying that "repetition is the mother of learning", to reduce the amount of memory work and avoid confusion, we should familiarize ourselves with two general concepts that are presented in the following two subsections, 1.15.2 and 1.15.3.

1.15.2. What Is Acidic Reacts with What Is Basic and What Is Basic Reacts with What Is Acidic. In the world of chemistry, an acid and a base are like two opposites. A base reacts with an acid and an acid reacts with a base. Moreover, what is not necessarily an acid per se yet is acidic (such as an acidic oxide or a hydro salt) is prone to react with what is basic. Likewise, what is not necessarily a metal hydroxide base yet is basic (such as a basic oxide or a hydroxo salt) is prone to react with what is acidic.

- Acids react with bases (metal hydroxides), basic oxides, and hydroxo salts.

- Bases (metal hydroxides) react with acids, acidic oxides, and hydro salts.

- Amphoteric hydroxides and oxides react with both acids and bases.

- Acidic oxides react with basic oxides.

As we go over the material in the next sections, try your best to (1) recognize acids, acidic compounds (acidic oxides and hydro salts), bases, and basic compounds (basic oxides and hydroxo salts) by their formulas, (2) note their reaction partners, and (3) think of what other basic and acidic substances they could react with.

1.15.3. Reactions That Go to Completion. Most of the reactions discussed above can go in only one direction. Such reactions are called irreversible. It is also often said that irreversible reactions are those that go to completion, meaning that the reaction proceeds until the reagents have been fully consumed. There are also reversible reactions that can go both forward and backward, as briefly mentioned above in subsection 1.12.7. These reactions obviously do not go to completion because the products formed in the forward reaction are transformed back in the reverse reaction. For a reversible chemical process, both the forward and backward reactions occur continuously, nonstop.

Figure 1-70 provides an example of one reaction that goes to completion and one reaction that does not. When a solution of AgNO3 is added to a solution of NaCl, a white precipitate of AgCl is formed right away. The AgCl can be separated by filtration and the other product, NaNO3, can be isolated by evaporation of the filtrate. If the reagents are mixed in the right 1:1 ratio, both will be completely consumed in the reaction.
Figure 1-70. One reaction (top) goes to completion, whereas the other (bottom) does not.

Repeating the experiment with KNO3 in place of AgNO3 does not produce any precipitate (Figure 1-70, bottom). The solution stays clear. Evaporation of the solution gives a mixture of all four salts, the two products (NaNO3 and KCl) and two starting materials (KNO3 and NaCl). This reaction does not go to completion.

In the next three subsections we will learn a considerable number of reactions involving acids, bases, and salts in solution, which all go to completion. But how do we know if a reaction goes to completion or not? Just remember the simple rules below.

A reaction in solution goes to completion if one of the products of the reaction is:

(1) water; or

(2) insoluble (or poorly soluble) and precipitates out; or

(3) a gas that escapes from the reaction mixture.

While water is easily recognizable by its formula, how do we know if there is a gas and/or precipitate produced in a reaction by looking at a given chemical equation? Upward and downward arrows after the formula of a product in a chemical equation serve an indication of a gas and a precipitate, respectively. But what do we do if no arrows are drawn?

There are not that many gases and insoluble solids that we need to memorize in order to feel comfortable learning and understanding most of the reactions discussed in the current course module.

Gases: hydrogen (H2); oxygen (O2); and carbon dioxide (CO2).

Precipitates commonly produced in reactions. Salts: silver chloride (AgCl) as well as bromide (AgBr) and iodide (AgI); barium and calcium sulfates (BaSO4 and CaSO4); and calcium carbonate (CaCO3). Metal hydroxides: all metal hydroxides, except NaOH, KOH, Ba(OH)2, and Ca(OH)2 (as well as the more exotic CsOH, RbOH, and LiOH). Examples include Cu(OH)2, Fe(OH)2, Fe(OH)3, Al(OH)3, and Zn(OH)2. Acids: silicic acid (H2SiO3), which often precipitates out of solution in the form of a gel (see next section and Volume 3).

1.15.4. Exercises.

1. Would you expect NaOH to react with (a) HCl; (b) Ca(OH)2; (c) MgO; (d) CO2; (e) Zn(OH)2; (f) H2SiO3? Write balanced chemical equations for the reactions that take place. Answer

2. Would you expect SO3 to react with (a) HCl; (b) Mg(OH)2; (c) CaO; (d) CO2; (e) Ag2O? Write balanced chemical equations for the reactions that take place. Answer

3. Of the reactions below identify those that go to completion.

(a) H2SO4 + Ba(OH)2 = BaSO4 + 2 H2O

(b) H2SO4 + BaCl2 = BaSO4 + 2 HCl

(c) 2 HNO3 + BaCl2 = Ba(NO3)2 + 2 HCl

(d) Ag2O + 2 HCl = 2 AgCl + H2O

(e) Ag2O + 2 HNO3 = 2 AgNO3 + H2O

(f) KBr + NaCl = KCl + NaBr

(g) 2 NaOH + H2S = Na2S + 2 H2O

(h) 2 H2O2 = 2 H2O + O2

(i) 2 HCl + MgO = MgCl2 + H2O

(j) 2 NaCl + Ba(OH)2 = 2 NaOH + BaCl2

(k) Ca(NO3)2 + K2CO3 = CaCO3 + 2 KNO3

(l) 2 Na + 2 H2O = 2 NaOH + H2

(m) ZnO + 2 NaOH = Na2ZnO2 + H2O

(n) CaCO3 + 2 HCl = CaCl2 + CO2 + H2O

(o) Zn + H2SO4 = ZnSO4 + H2

(p) Cu(OH)2 + 2 HNO3 = Cu(NO3)2 + 2 H2O